Subsidence can be caused by: certain soils (clay soils are particularly vulnerable to subsidence since they shrink and swell depending on their moisture content), trees and shrubs taking moisture from soils causing them to shrink, leaking drains, which can soften or wash away the ground beneath the foundations and, less commonly, where properties are built over, or close to, mine workings.
The first obvious sign of subsidence is the appearance of cracks. These will normally be small diagonal cracks, about the thickness of a 10p coin, which appear suddenly in plasterwork and outside brickwork. Doors and windows may also start to stick due to the distortion of the building.
Most buildings experience some minor cracking at one point or another due to changes in temperature and humidity. Most minor cracks are nothing to worry about.
Bars, ground screws and flexible grout are used to restore the structural integrity of the wall. These devices distribute the forces evenly along the wall reducing the possibility of mirror cracking later on.